The present invention relates to a support boat (wafer boat) for objects to be processed which is used in a vertical heat processing apparatus for heat treating semiconductor wafers, etc.
Generally, various heat treatment apparatuses are used in oxidizing the surfaces of objects to be processed, such as semiconductor wafers, glass substrates of amorphous Si for use in LCDs, etc., and in forming diffused layers, silicon oxide films, silicon nitride films, etc. on the surfaces of the objects to be processed.
As one of such heat treatment apparatuses is noted as "fast thermal process" (abbreviated as FTP) which can fast increase/decrease temperatures for the purpose of suppressing as much as possible formation of natural oxide films on objects to be processed which are immediately before or after treatments to thereby meet demand for improved characteristics of formed films.
The FTP can achieve temperature increasing speeds and temperature decreasing speeds of about 100.degree. C./min which is higher in contrast to, e.g., 3-4.degree. C. temperature increasing and decreasing speeds of the usual heat treatment apparatuses, and can form films of higher quality.
For a treatment, semiconductor wafers are loaded into a processing furnace, mounted on a support boat for supporting a number of wafers horizontal on multi-stages spaced from each other by a certain vertical pitch. What is a problem upon fast temperature increase is how to increase temperatures, maintaining the temperatures uniform in the wafer surfaces. That is, temperatures tend to rise faster at peripheral parts of the wafers near a heater for heating, and the temperatures tend to delay in rising at central parts of the wafers remote from the heater. To prevent such phenomena, wafers are supported by ring-shaped circular support shelves for the purpose of suppressing the faster temperature rise of the peripheral parts of the wafers.
Such temperature increase of wafers will be explained with reference to FIGS. 7 and 9.
FIG. 7 is a schematic vertical sectional view of the generally used in FTPs. FIG. 8 is an enlarged partial view of the support boat for wafers. A processing vessel 2 has the bottom opened and has a single tubular or a double tubular structure. Wafers W are loaded in the processing vessel 2, mounted horizontal on multi-stages of a support boat of, e.g., quartz or SiC, which are spaced from each other by a prescribed vertical pitch. Peripheral parts of the wafers W are supported by the support boat 4 at, e.g., 4 or 6 points. The support boat 4 is mounted on a lid 8 through a heat insulating cylinder 6. The opening in the bottom of the processing vessel 2 is closed tight by the lid 8 for a treatment. The lid 8 is connected to a lift mechanism 10, such as a boat elevator, and moves the support boat 4 up and down into and out of the processing vessel 2 when the wafers are transferred.
A number of heaters 14 for heating mounted on the inside wall of a cylindrical heat insulating member 12 are disposed around the processing vessel 2, and heat the processing vessel 2 at the side thereof.
Especially the heaters 14 are not usual ones but special heaters of molybdenum disilicate or others, which provides high calories per a unit area, so as to enable the fast temperature increase.
By the use of such special heaters having high calories per a unit area as the heaters 14, the fast temperature increase of the treatment apparatus can be attained as described above, but peripheral parts of the wafers near the heaters 14 tend to faster increase temperatures than central parts thereof remote from the heaters 14. As a result, it is impossible to increase temperatures, maintaining the temperatures uniform in the surfaces of the wafers.
As shown in FIG. 8, ring-shaped support shelves 16 are provided in support bars 4A of the support boat 14, and peripheral parts of the wafers W are held on the support shelves 16. Because of a certain heat capacity of the support shelves 16, temperature increase speeds here are a little mitigated, and resultantly the temperature increase is conducted, retaining temperatures uniform in the surfaces of the wafers W.
A state of the wafers during the temperature increase is shown in FIG. 9A. FIG. 9A shows a temperature distribution in the surfaces of the wafers during the temperature increase. In FIG. 9A, the curve A indicates a temperature distribution of the support shelves supporting the wafers W at three points. Peripheral parts of the wafers have a larger temperature increase, and central parts of the wafers have a considerably lower temperature in comparison with the peripheral parts. The curve B indicates a temperature distribution of the wafers at the ring-shaped support shelves, and shows the temperature increase of a preferable characteristic that a heat capacity of the ring-shaped support shelves suppresses the temperature increase of the peripheral parts of the wafers to some extent.
By the use of the ring-shaped support shelves to support the wafers W as described above, the temperature increase on the peripheral parts can be suppressed to obtain the improved temperature characteristic on most of the wafers as explained in FIG. 9A. Inversely on some sheets of the wafers W at the upper and the lower ends of the support boat, as shown in FIG. 9B, the temperature increase is larger at the central parts of the wafers than at the peripheral parts thereof. Inversion of the heat distribution takes place with respect to the distribution of FIG. 9A.
A cause for such inversion of the heat distribution is that heat from the heaters to the central parts of the wafers tends to be blocked by those of the wafers which are not positioned near the upper end of the support boat and are vertically adjacent to each other, and fail to reach those wafers. Those of the wafers near the upper end of the support boat are generally subjected to larger calories because the upper end of the support boat is exposed to the heaters. Accordingly the ring-shaped support shelves inversely acts to cause the inversion of the temperature distribution shown in FIG. 9B.
FIG. 10 is a schematic view of the heat conduction near the upper end of the support boat in this time. In FIG. 10 direct heat from the heaters 14 is indicated by the broken arrows, and the heat conduction between the wafers is indicated by the wavy arrows. As apparent in FIG. 10, first the top end of the support boat is heated, and the heat is conducted to the first top one of the wafers and the support shelf supporting the first top wafer and sequentially to the second top one and to the third top one. The heat is not easily conducted to the central parts of ones of the wafers on the intermediate support shelves in comparison with those of the wafers on upper support shelves, and accordingly a proper temperature distribution is gradually formed.
Thus the inversion of the heat distribution occurs on some of the wafers at the upper ends of the support boat and have a heat history different from that of the rest wafers. This is unfavorable to electric characteristics. For preventing this occurrence, a number of dummy wafers, which are unusable as product wafers must be mounted on the side of the upper end of the support boat, which not only lowers throughputs, but also needs an accordingly longer furnace length of the heat treatment apparatus. These are problems.